| 1. |
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| 2. |
- Brenning, Nils, et al.
(author)
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Interpretation of the Electric Fields Measured in an Ionospheric Critical Ionization Velocity Experiment
- 1991
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96, s. 9719-9733
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Journal article (peer-reviewed)abstract
- This paper deals with the quasi-dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV) mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three-dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic-field-aligned component of the electric field; the quasi-dc electric field observed within the streams had a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes. One interpretation of the quasi-dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic-field-aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence-free. It is argued that this interpretation can explain both a reversal of the strong explosion-directed electric field in burst 1 and the absence of such a reversal in burst 2.
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| 3. |
- Cumnock, Judy, et al.
(author)
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Response of the Ionospheric Convection Pattern to a Rotation of the Interplanetary Magnetic Field on January 14, 1988
- 1992
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 97, s. 19449-19460
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Journal article (peer-reviewed)abstract
- Ionospheric convection signatures observed over the polar regions are provided by the DMSP F8 satellite. We consider five passes over the southern summer hemisphere during a time when the z component of the interplanetary magnetic field was stable and positive and the y component changed slowly from positive to negative. Large-scale regions of sunward flow are observed at very high latitudes consistent with a strong z component. When B(y) and B(z) are positive, but B(y) is greater than B(z), strong evidence exists for dayside merging in a manner similar to that expected when B(z) is negative. This signature is diminished as B(y) decreases and becomes smaller than B(z) resulting in a four-cell convection pattern displaced toward the sunward side of the dawn-dusk meridian. In this case the sign of B(y) affects the relative sizes of the two highest-latitude cells. In the southern hemisphere the dusk side high-latitude cell is dominant for B(y) positive and the dawnside high-latitude cell is dominant for B(y) negative. The relative importance of possible electric field sources in the low-latitude boundary layer, the dayside cusp, and the lobe all need to be considered to adequately explain the observed evolution of the convection pattern.
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| 4. |
- Elgered, Gunnar, 1955, et al.
(author)
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Geodesy by Radio Interferometry: Water Vapor Radiometry for Estimation of the Wet Delay
- 1991
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96:B4, s. 6541-6555
-
Journal article (peer-reviewed)abstract
- An important source of error in very-long-baseline interferometry (VLBI) estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. We present and discuss the method of using data from a water vapor radiometer (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The lengths of the baselines range from 919 to 7941 km. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13% smaller weighted-root-mean-square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the “best” minimum elevation angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass. For use of WVR data along with accurate determinations of total surface pressure, the best minimum is about 20 degrees; for use of a model for the wet delay based on the humidity and temperature at the ground, the best minimum is about 35 degrees.
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| 5. |
- Knipp, D.J., et al.
(author)
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Ionospheric Convection Response to Slow, Strong Variations in a Northward Interplanetary Magnetic Field: A Case Study for January 14, 1988
- 1993
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 19273-19292
-
Journal article (peer-reviewed)abstract
- We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF B(y) and B(z) components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF B(x) component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio \textbackslashB(y)/B(z)\textbackslash exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of B(y) exceeded that of B(z), with potential drops of the order of 100 kV, even while B(z) remained northward. The later transition to southward B(z) produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through \textbackslashB(y)/B(z)\textbackslash = 1 than at the transition through B(z) = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.
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| 6. |
- Lönnqvist, Håkan, et al.
(author)
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Generation of VLF saucer emissions observed by the Viking satellite
- 1993
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 13565-13574
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Journal article (peer-reviewed)abstract
- Simultaneous observations by the Viking satellite of electric and magnetic fields as well as charged particles have been used to investigate V-shaped wave phenomena. The intensity of these VLF and ELF emissions is V-shaped when shown in a frequency versus time plot. Simultaneous observations of V-shaped so-called VLF saucer emissions, particles and field-aligned currents strongly suggest, for the first time, that upgoing electrons with energies less than a few hundred electron volts can generate these waves. Broadband waves observed inside the saucer generation region, form frequencies much less than the ion cyclotron frequency up to the plasma frequency, may also be generated by these electrons. Viking observations of VLF saucers at altitudes between 4000 km and 13,500 km show that these emissions occur at higher altitudes tha discussed in previous reports. The generation regions seem to be more extended at these higher altitudes than what has been reported at lower altitudes by other observations.
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| 7. |
- Marklund, Göran, et al.
(author)
-
On the electrodynamic state of the auroral ionosphere during northward interplanetary magnetic field - a transpolar arc case study
- 1991
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96:A6, s. 9567-9578
-
Journal article (peer-reviewed)abstract
- The ionospheric electrodynamical state has been reconstructed for a transpolar arc event during northward interplanetary magnetic field conditions. An extensive set of observations by Viking and other satellites and by ground-based radars has been used to provide realistic model input data or to verify the modeling results. The resulting convection pattern is found to be consistent with the Viking electric field and intimately linked to the prevalent auroral distribution. It is characterized by a large evening cell, well extended across noon and split up by two separated potential minima, and a minor crescent-shaped morning cell. The convection signatures are found to vary a lot along the transpolar arc depending on the relative role of the arc-associated convection and the ambient convection. The transpolar arc is generally embedded in antisunward convective flow except near the connection points with the auroral oval, where sunward flow exists in localized regions.
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| 8. |
- Marklund, Göran, et al.
(author)
-
On the influence of localized electric fields and field-aligned currents associated with polar arcs on the global potential distribution
- 1991
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96:A8, s. 13977-13983
-
Journal article (peer-reviewed)abstract
- The influence of localized field-aligned current, associated with intense transpolar arcs mostly occuring during periods of northward interplanetary magnetic field (IMF), on the global electrodynamics has been investigated using a numerical simulation model. Idealized field-aligned current distributions representing both the region 1/2 system of the auroral oval and the transpolar arc as well as a corresponding ionospheric conductivity distribution are fed into the model to calculate the potential distributions. The transpolar arc has been represented by a few alternative field-aligned current distributions which are different in the way the downward return currents are distributed in the ionosphere. For the case with a single, upward current sheet the potential pattern assumes a form similar to that typical for IMF B(y) positive conditions, namely a large dusk cell with sunward drift reaching very high latitudes and a crescent-shaped dawn cell. If the conductivity of the main auroral oval is comparable to that of the polar arc the dusk cell will have two local potential minima and thus a region of weak antisunward convection in between. For the cases with two equal but oppositely directed current sheets the potential patterns are very similar to the symmetrical two-cell reference pattern associated with solely the region 1/2 system with an exception for the immediate vicinity of the theta aurora. Depending on the direction of the polar arc current sheets the dawn-dusk electric field will either be reversed (or weakened) or intensified at the location of the transpolar arc. The presence of a reversal depend, however, not only on the relative magnitude between the polar arc currents and those of the region 1/2 system but also on the characteristics of the acceleration region and of the conductivity distribution associated with the polar arc. Comparisons are made between the model results and Viking electric field data for a number of polar arc crossings to reveal the most common electrodynamical signatures of these auroral phenomena.
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| 9. |
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| 10. |
- Pedersen, Laust Börsting, et al.
(author)
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Electric resistivity in the Gravberg-1 deep well, Sweden
- 1992
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 97:6B, s. 9171-9182
-
Journal article (peer-reviewed)abstract
- The Siljan impact structure located in central Sweden has been the object of intense geophysical and geological studies for several decades. This paper presents the results of electrical resistivity measurements on the surface, in the hole, and on core samples from shallow coreholes distributed over the whole impact structure. Magnetotelluric data constrain the central part of the structure to be essentially one-dimensional with an upper crustal unit of about 10 000 Ωm followed by a less resistive impact related unit of 1000 Ωm from a depth of about 6 km to a depth of 20 km. The lower crust is a homogeneous unit of about 300 Ωm extending down to about 40 km, where the upper mantle is marked by an increase in resistivity of about 1000 Ωm. Major fracture zones, some of which coincide with the horizontal dolerite sills, known to extend over tens of kilometers, are probably the main carriers of current. Saline fluids recovered below 5.4-km depth and the magnetotelluric results lead us to conclude that pore fluids are highly saline below this depth even on a regional scale.
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| 11. |
- Rachidi, Farhad, et al.
(author)
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Determination of lightning currents from far electromagnetic fields
- 1993
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 98:D10, s. 18315-18321
-
Journal article (peer-reviewed)abstract
- Expressions relating far electromagnetic fields and return stroke channel base currents have been derived in the literature for various lightning return stroke models. The use of such relations permits the estimation of channel base currents of return strokes, testing of the individual models using sets of simultaneous current and field measurements, and also the determination of nondirectly measurable parameters of the models. In this paper we derive a relation between the far electromagnetic field and the channelbase current for the Diendorfer-Uman (DU) and the modified Diendorfer-Uman (MDU) models. Additionally, the current field relations for all models for which such an analytical relation can be derived are summarized. Finally, a numerical solution technique is proposed to solve the equations relating the far field and channel base current of the DU and the traveling current source (TCS) models.
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| 12. |
- Rakov, Vladimir A., et al.
(author)
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Electric field pulses in K and M changes of lightning ground flashes
- 1992
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 97:D9, s. 9935-9950
-
Journal article (peer-reviewed)abstract
- From electric field records of 27 ground flashes near Tampa, Florida, and 19 ground flashes at the NASA Kennedy Space Center (KSC) the occurrence and waveshape of microsecond-scale electric field pulses associated with both millisecond-scale steplike K changes and millisecond-scale hook-shaped M changes are examined to test and disprove the following two hypotheses: (1) that K changes contain a microsecond-scale pulse component which can be described by the characteristic pulse waveform proposed by Arnold and Pierce (1964) and (2) that there is essentially no difference between K and M processes, as argued by Kitagawa et al. (1962). Microsecond-scale electric field variations exceeding by at least 50% the system noise level were observed in 23% of 135 K changes from Tampa and in 25% of 128 K changes from KSC, while such field variations were found in 44% of 88 M changes from Tampa and in 77% of 30 M changes from KSC. In the majority of the K changes having microsecond-scale pulse activity, that activity did not occur at the beginning of the K step, while in most cases the pulses associated with M changes occurred at the initial portion of the M hook. These results can be interpreted to imply that K changes and M changes are associated with dissimilar physical processes, in refutation of hypothesis (2) above. The microsecond-scale pulse activity during K changes and M changes was highly variable and sometimes irregular in waveshape. Not all the pulses had the same polarity as the K step or the initial portion of the M hook on which they were superimposed. The relation of the microsecond-scale variations to the overall K changes in ground flashes as regards the frequency of occurrence, the position of pulses within the slower field change, and the shape of the pulses is similar to that reported by Bils et al. (1988) for K changes in cloud flashes. The observed microsecond-scale field variations associated with K changes are not consistent with the characteristic electric field pulse waveform attributed by Arnold and Pierce (1964) and some other investigators to K changes, in refutation of hypothesis (1) above. No relation was observed between the magnitude of a K change and the presence or absence of corresponding microsecond-scale field variations. M changes during continuing-current field changes of relatively short duration (less than 20 ms or so) are more likely to have pulses than M changes during continuing-current field changes of longer duration.
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| 13. |
- Rakov, Vladimir A., et al.
(author)
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On the empirical formula of Willett et al. relating lightning return-stroke peak current and peak electric field
- 1992
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 97:D11, s. 11527-11533
-
Journal article (peer-reviewed)abstract
- The empirical formula proposed by Willett et al. (1989) for the estimation of lightning return-stroke peak current, I, from measured peak electric field, E, at a range D, is analyzed and discussed. The formula of Willett et al. (1989), obtained from the regression of E on I, is not the least squares fit and hence is not the best expression for predicting the peak current from the measured peak electric field. Based on the same data, the least squares fit and, hence, the best expression is obtained from the regression of I on E, given by I = 1.5 − 0.037DE, where I is in kA and taken as negative, E is positive and in V/m, and D is in km. The Willett et al. (1989) formula results in an error with respect to the best peak current estimating expression that varies from −15% to +2.6% over the range of peak field values of 1.9 to 11 V/m (normalized to 100 km) used to derive the two relations. When the data of Willett et al. (1989) are separated into a high returnstroke speed group (1.5 × 108 to 1.9 × 108 m/s) and a low return-stroke speed group (1.2 × 108 to 1.4 × 108 m/s), the I-E regression lines differ for the two groups, with the difference in the regression line slopes being statistically significant at the 0.01 significance level. If the difference between intercepts of these two regression lines, found to be statistically insignificant, is neglected, the observed difference in slopes suggests that the group with the higher measured return-stroke speed is associated with a lower peak electric field for the same peak current. Finally, the practical applications of the Willett et al. (1989) formula presently found in the literature are reviewed, and the several cases of improper use, mostly related to misinterpretation of Willett et al. ’s (1989) sign convention, are corrected.
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| 14. |
- Rothwell, P.L., et al.
(author)
-
Acceleration and Stochastic Heating of Ions Drifting Through an Auroral Arc
- 1992
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 97, s. 19333-19339
-
Journal article (peer-reviewed)abstract
- We find that ions E x B drifting through an auroral arc can undergo transverse acceleration and stochastic heating. This result is very analogous to recent work regarding similar phenomena in the magnetotail (Buchner and Zelenyi, 1990; Chen and Palmadesso, 1986; Brittnacher and Whipple, 199 1). An analytic expression for the maximum arc width for which chaotic behavior is present is derived and numerically verified. We find, for example, that a 1.5-km-thick arc at LAMBDA = 65-degrees requires a minimum potential drop of 3 kV for transverse ion acceleration and heating to occur. Thicker arcs require higher potential drops for stochasticity to occur. This mechanism could be a source for conic ions.
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| 15. |
- Rothwell, P.L., et al.
(author)
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O+ phase bunching and auroral arc structure
- 1994
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 99, s. 2461-2470
-
Journal article (peer-reviewed)abstract
- The equations of motion are solved for ions moving in a model electric field that corresponds to the nightside equatorial region df the magnetosphere. The model represents the poleward region of the Harang discontinuity mapped to the magnetosphere. Within this region the model electric field has a constant earthward gradient superimposed on a constant dawn-to-dusk electric field. In combination with the earthward drift motion due to the dawn-to-dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O+ ions than for H+ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The striations are enhanced for more abrupt changes in the electric field gradient, a weaker magnetic field, a stronger cross-tail electric field and colder O+ ions. The first two conditions apply during the growth phase of a substorm. Using the Tsyganenko (1987) model a minimum electric field gradient value of 1 x 1O(-9) V/m(2) ((1 mV/m)/1000 km) at L = 6-7 is found. Charge neutrality requires coupling with the ionosphere through electrons moving along magnetic held lines, and such electrons may be the cause of multiple auroral arcs.
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| 16. |
- Rothwell, P.L., et al.
(author)
-
PREBREAKUP ARCS - A COMPARISON BETWEEN THEORY AND EXPERIMENT
- 1991
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96, s. 13967-13975
-
Journal article (peer-reviewed)abstract
- We have developed a model describing the structure of a prebreakup arc based on an ionospheric Cowling channel and its extension into the magnetosphere. A coupled two-circuit representation of the substorm current wedge is used which is locally superimposed on both westward and eastward electrojets. We find that brighter, more unstable prebreakup arcs are formed in the premidnight (southwest of the Harang Discontinuity) than in the postmidnight (northeast of the Harang Discontinuity) sector. This contributes to the observed prevalence of auroral activity in the premidnight sector. Also, our model predicts that the north-south dimensions of the current wedge in the ionosphere should vary from a few kilometers at an invariant latitude (LAMBDA) of 62-degrees to hundreds of kilometers above LAMBDA = 68-degrees. Comparison of the model results with the extensive observations of Marklund et al. (1983) for a specific arc observed just after onset shows good agreement, particularly for the magnitude of the polarization electric field and the arc size. We conclude that this agreement is further evidence that the substorm breakup arises from magnetosphere-ionosphere coupling in the near magnetosphere and that the steady state model developed here is descriptive of the breakup arc before inductive effects become dominant.
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| 17. |
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| 18. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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Comparison of lightning return-stroke models
- 1993
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 98:D12, s. 22903-22914
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Journal article (peer-reviewed)abstract
- Five return-stroke models, each allowing the use of measured channel-base current and return-stroke speed as inputs for the computation of channel current distribution and remote electric field, are compared and evaluated using 18 sets of three simultaneously measured triggered lightning features: channel-base current, return-stroke speed, and electric field at a distance of about 5 km from the channel base. The experimental data were acquired during a triggered lightning experiment at the NASA Kennedy Space Center, Florida, in 1987 and were reported in part by Willett et al. (1989) and Leteinturier et al. (1991). The models compared are the transmission line (TL) model, the modified transmission line (MTL) model, the traveling current source (TCS) model, the Diendorfer-Uman (DU) model, and the modified Diendorfer-Uman (MDU) model. The TL, MTL, DU, and MDU models each predict the measured initial electric field peaks with an error whose mean absolute value is about 20%; the TCS model has a mean absolute error about twice that value. For the prediction of overall measured field wave shape, none of the models is clearly preferred, although for the model parameters assumed, the MDU model gave the best wave shape match. Most of the return strokes that exhibited very narrow sharp initial peaks in the measured electric field waveforms had a maximum rate of rise of channel-base current closer to the peak of the measured channel-base current waveform than did return strokes not exhibiting these sharp field peaks. The calculated fields from the TL and the MTL models do not have narrow sharp peaks similar to those found in many of the measured fields, while the fields calculated from the TCS, DU, and MDU models had somewhat similar peaks in most of the cases where those peaks were found in the measured fields. On the basis of the comparison of the five models, we recommend the TL model for calculating the peak channel-base current from the measured initial peak electric field because the TL model provides a similar or better result from a simpler mathematical relation.
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| 19. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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Extension of the Diendorfer-Uman lightning return stroke model to the case of a variable upward return stroke speed and a variable downward discharge current speed
- 1991
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 96:D9, s. 17143-17150
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Journal article (peer-reviewed)abstract
- A new lightning return stroke model has recently been proposed by Diendorfer and Uman (1990). In this model, if one specifies a current at the channel base (ground), a constant return stroke speed, and a channel discharge time constant, one can derive analytically the current and charge as a function of time and height associated with the channel above ground. Here we present a more general and more straightforward derivation of the Diendorfer-Uman model. In the new formulation we allow a variable return stroke speed that can be any arbitrary function of height. The influence of a decrease in speed with height, as occurs in nature, on the channel current and charge distributions and on the radiated electric and magnetic fields is determined and compared with the constant speed case. For a given channel base current, a decreasing speed with height does not change the initial peak electric and magnetic fields appreciably from the values found for a constant speed having the same value as the variable speed at the channel base, but a decreasing speed can cause considerable changes in field waveshapes for both the distant radiation fields and the near electrostatic fields. In addition to allowing an arbitrary return stroke speed, the new formulation of the Diendorfer-Uman model allows the downward propagating current waves released by the return stroke front also to have an arbitrary variable speed, whereas the original model assumed that speed to be constant at the speed of light.
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| 20. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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K and M changes in close lightning ground flashes in Florida
- 1990
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 95:D11, s. 18631-18640
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Journal article (peer-reviewed)abstract
- Electric field changes produced by K and M processes in Florida lightning ground flashes at distances within 12 km are analyzed and compared. The geometric-mean time durations were similar: 0.7 ms for 135 K changes and 0.9 ms for 80 M changes. The geometric-mean time intervals between events were dissimilar: 12.5 ms for 93 K change intervals and 2.1 ms for 48 M change intervals. Our geometric mean K change interval is about twice the median value observed by Kitagawa et al. (1962) in New Mexico who used a somewhat different measurement technique, while our geometric mean M change interval is about one-third the New Mexico median value although Kitagawa et al. (1962) apparently excluded from their statistical data those M components occurring within 15 ms of the return stroke initial peak field, and we included these. The K electric field changes were not always simple ramps superimposed on the slower interstroke J or final F field changes, but, rather, could be grouped into four general waveshape categories. All M field changes had relatively similar hooked shapes. We argue that the hypothesis proposed by Kitagawa et al. (1962) that K and M processes are essentially the same physical phenomenon is not supported by our data, although it cannot be positively ruled out; and, additionally, that the conclusion of Kitagawa and Brook (1960) that the net interstroke field change (J change) is essentially the sum of the individual K changes is also not supported by our data. Further, our data suggest that there is a significantly higher upper limit for the moment change associated with K processes in ground flashes than the 1 to 2 C-km reported by Brook and Kitagawa (1960) and Ogawa and Brook (1964).
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| 21. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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Lightning return stroke model with height-variable discharge time constant
- 1994
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 99:D11, s. 22773-22780
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Journal article (peer-reviewed)abstract
- A new lightning return stroke model is proposed in which the lightning channel, previously charged by the leader, is exponentially discharged with the discharge time constant being a general function of height. The new model is both more general and more physically reasonable than the Diendorfer-Uman model (Diendorfer and Uman, 1990) and its modification by Thottappillil et al. (1991). In the new model a single height-variable discharge time constant is used instead of the two height-invariant discharge time constants used for natural lightning in the Diendorfer-Uman model. As an illustration of the use of the new model, the measured electric fields for three triggered lightning return strokes about 5-km distant are compared with the fields calculated from the model, using simultaneously measured channel base current and return stroke speed as inputs to the model.
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| 22. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
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Lightning subsequent-stroke electric field peak greater than the first stroke peak and multiple ground terminations
- 1992
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 97:D7, s. 7503-7509
-
Journal article (peer-reviewed)abstract
- For 46 multiple-stroke flashes in which each stroke ground termination was located using a TV camera network and thunder ranging, 15 flashes (33%) had one or more subsequent return strokes whose initial electric field peak normalized to 100 km was greater than the first-stroke field peak of the flash. In 9 of these 15 flashes the subsequent strokes with field peaks greater than the first stroke followed the same channel as the first stroke; in five flashes the subsequent strokes with the greater peaks followed a different channel to ground; and in one flash the subsequent strokes with the greater peaks occurred both in the first-stroke channel and in a different channel. The interstroke intervals immediately preceding the 13 larger subsequent strokes that followed the first-stroke channel had a geometric mean (GM) duration of 98 ms, 1.7 times greater than the GM of 57 ms for all 199 interstroke intervals (46 flashes) without any selection. Eight of the 13 larger subsequent strokes for which leader durations were measurable had a GM leader duration of 0.55 ms, 3.3 times smaller than the GM of 1.8 ms for 117 subsequent leaders with measurable duration in a previously formed channel of the 46 multiple-stroke flashes. For the six larger subsequent strokes that created a new channel to ground, the preceding interstroke interval had a GM of 130 ms, and the leader duration had a GM of 15 ms. No subsequent stroke with peak field exceeding the first in any category had a preceding interstroke interval less than 35 ms. Analysis of direct current measurements from Switzerland shows that subsequent-stroke currents exhibit many features similar to those of Florida subsequent-stroke electric fields. In 22 Florida single-stroke and multiple-stroke ground flashes the distances between multiple channel terminations in a given flash (33 measurements) ranged from 0.3 km to 7.3 km, with a GM of 1.7 km.
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| 23. |
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| 24. |
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| 25. |
- DUBOULOZ, N, et al.
(author)
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DETAILED ANALYSIS OF BROAD-BAND ELECTROSTATIC NOISE IN THE DAYSIDE AURORAL-ZONE
- 1991
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In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - SWEDISH INST SPACE PHYS,S-75590 UPPSALA,SWEDEN. ROYAL INST TECHNOL,S-10044 STOCKHOLM 70,SWEDEN.. - 0148-0227. ; 96:A3, s. 3565-3579
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Journal article (peer-reviewed)abstract
- The great number of bursts of broadband electrostatic noise (BEN) recorded during crossings of the dayside auroral zone by the Viking satellite enables their statistical study. The angular distribution of BEN with respect to the Earth's magnetic field is shown to be most likely isotropic, implying that it cannot consist of a unique linear plasma mode. Most of the bursts evidence a power law spectrum from the lower hybrid and ion plasma frequencies up to frequencies sometimes much higher than the electron plasma frequency, suggesting the presence of nonlinear effects. This is confirmed by their high intensity, and by the correlation between their amplitude and their frequency extension. BEN emissions are associated with ion conical distributions and with field-aligned electron beams. Although most of the power is concentrated at very low frequencies and around the lower hybrid and ion plasma frequencies, electron acoustic and beam mode waves may contribute to the high-frequency extension of BEN. The most intense BEN emissions are also correlated with sharp cold plasma density gradients and probably involve drift instabilities. Strong quasi-static perpendicular electric fields, which induce high-speed plasma flows, are also measured, so that the Doppler effect may contribute to the broadening of the BEN spectrum.
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| 26. |
- Edner, H, et al.
(author)
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Differential Absorption Lidar Mapping of Atmospheric Atomic Mercury In Italian Geothermal Fields
- 1992
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In: Journal of Geophysical Research. - 2156-2202. ; 97:D4, s. 3779-3786
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Journal article (peer-reviewed)abstract
- Results from extensive lidar measurements on atmospheric atomic mercury in Italian geothermal fields are reported. A mobile differential absorption lidar system operating on the 254-nm mercury resonance line with a measuring range of about 1 km was used in mineralized as well as nonmineralized areas. Measurements were performed at geothermal power stations and in an unexploited field with natural surface geothermic manifestations. Atomic mercury concentrations ranging from 2 to 1000 ng/m3 were mapped. The high Italian geothermal mercury concentrations are in strong contrast to the recent lidar finding of the absence of atomic mercury in Icelandic geothermal fields.
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| 27. |
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| 29. |
- HULTQVIST, B, et al.
(author)
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ON THE UPWARD ACCELERATION OF ELECTRONS AND IONS BY LOW-FREQUENCY ELECTRIC-FIELD FLUCTUATIONS OBSERVED BY VIKING
- 1991
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In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : American Geophysical Union (AGU). - 0148-0227. ; 96:A7, s. 11609-11615
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Journal article (peer-reviewed)abstract
- The dependence of the generation of upward electron beams in the to side ionosphere on the amplitude of the slow electric field fluctuations and the magnitude of the dc potential difference in the acceleration region is found to agree with the model involving the fluctuating electric field component presented in 1988 by Hultqvist. The ratio between the amplitude of the electric field fluctuations, E(f), determined by the dipole electric field experiment, and the dc acceleration voltage V(acc), given by the energy of the upflowing ion beam, is much higher on average for satellite spins showing an upward electron beam than for the spins without. It is also shown that low plasma density favors the generation of electron beams.
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| 31. |
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| 32. |
- Olsson, J., et al.
(author)
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Fractal analysis of high-resolution rainfall time series
- 1993
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In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227. ; 98:D12, s. 23265-23274
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Journal article (peer-reviewed)abstract
- Two-year series of 1-min rainfall intensities observed by rain gages at six differrent points are analyzed to obtain information about the fractal behavior of the rainfall distribution in time. First, the rainfall time series are investigated using a monodimensional fractal approach (simple scaling) by calculating the box and correlation dimensions, respectively. The results indicate scaling but with different dimensions for different time aggregation periods. The time periods where changes in dimension occur can be related to average rainfall event durations and average dry period lengths. Also, the dimension is shown to be a decreasing function of the rainfall intensity level. This suggests a multidimensional fractal behavior (multiscaling), and to test this hypothesis, the probability distribution/multiple scaling method was applied to the time series. The results confirm that the investigated rainfall time series display a multidimensional fractal behavior, at least within a significant part of the studied timescales, which indicates that the rainfall process can be described by a multiplicative cascade process. -Authors
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| 33. |
- Rakov, Vladimir A., et al.
(author)
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Review of lightning properties from electric field and TV observations
- 1994
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In: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-8996. ; 99:D5, s. 10745-10750
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Journal article (peer-reviewed)abstract
- From analysis of simultaneous electric field and TV records of 76 negative cloud-to-ground lightning flashes in Florida, various lightning properties have been determined and several new facets of lightning behavior inferred. Only 17 % of the flashes were single-stroke flashes, less than half the commonly claimed percentage (e.g., Anderson and Eriksson, 1980). The initial electric field peak (and, by inference, current peak) for the only strokes in single-stroke flashes was smaller than for first strokes in multiple-stroke flashes. Half of all flashes, single and multiple stroke, struck ground at more than one point, with the spatial separation between the channel terminations being up to many kilometers. One third of multiple-stroke flashes had at least one subsequent stroke whose distance-normalized initial electric field peak exceeded that of the first stroke in the flash. Thus such flashes are not unusual, contrary to the implication of most lightning protection and lightning test standards. Subsequent strokes of the order of 2 through 4 were more likely to create a new channel termination on ground than strokes of the order of 5 and higher. Further, leaders of lower-order subsequent strokes following previously formed and not-too-aged (100 ms or less) channels were more likely to show stepping, as opposed to continuous propagation (i.e., to be dart-stepped leaders rather than dart leaders), than were leaders of higher-order strokes. Finally, lower-order subsequent return strokes exhibited a larger initial electric field peak than did higher-order strokes. The second leader of the flash (the first subsequent leader) encounters the least favorable propagation conditions of all subsequent strokes: more than half of the second leaders either deflected from the previously formed path to ground or propagated in a stepped, as opposed to a continuous, fashion along the lowest part of that path. It is important to note that interstroke intervals preceding second strokes are similar to or shorter than those preceding higher-order strokes. These observations indicate that channel conditions for the propagation of a subsequent leader are determined not just by the immediately preceding channel heating and cooling processes but rather by the entire channel history. In particular, the status of the channel apparently depends on the number of strokes that have participated in its cumulative conditioning. The overwhelming majority of long continuing currents, those with a duration longer than 40 ms, were initiated by subsequent strokes of multiple-stroke flashes as opposed to either the first stroke in a multiple-stroke flash or the only stroke in a single-stroke flash. Strokes that initiate such long continuing currents were (1) relatively small (in terms of both return-stroke field peak and, as determined from an independent study in New Mexico, stroke charge), (2) followed relatively short interstroke intervals, and (3) showed a tendency to be preceded by a relatively large stroke. Millisecond-scale K and M electric field changes appeared different in terms of both microsecond-scale pulse content and interevent time intervals. Often no microsecond-scale K and M field pulses were detected. When they were present, such pulses were highly variable and sometimes irregular in waveshape, as opposed to the alleged characteristic K-pulse waveform described by Arnold and Pierce (1964), which has been extensively used in atmospheric radio-noise studies. There is a remarkable similarity between many lightning characteristics in Florida and in New Mexico.
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